🔭 astrophysics

The causal effect of cosmic filaments on dark matter halos

By employing a splicing method to isolate non-linear environmental effects, this study demonstrates that while proximity to cosmic filaments does not alter the mass or size of Milky Way-mass dark matter halos, it significantly impacts their spin and shape orientation, indicating that non-linear couplings to the large-scale environment are as critical as linear effects for accurate halo modeling and intrinsic alignment predictions.

Original authors: Anatole Storck, Corentin Cadiou, Oscar Agertz, Daniela Galárraga-Espinosa

Published 2026-03-20

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CC BY 4.0

Original authors: Anatole Storck, Corentin Cadiou, Oscar Agertz, Daniela Galárraga-Espinosa

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Question: Nature vs. Nurture for Galaxies

Imagine the universe as a giant, invisible web made of dark matter. This web has thick "roads" called filaments, and at the intersections or along these roads, giant clumps of dark matter form called halos. Our galaxies (like the Milky Way) sit inside these halos like passengers in a car.

Scientists have long wondered: How much of a galaxy's shape and spin is decided by its own "DNA" (the conditions when it was born), and how much is decided by its "neighborhood" (the cosmic web around it)?

Usually, scientists can't answer this because every galaxy is born in a slightly different neighborhood. It's like trying to figure out if a plant grows tall because of its seeds or because of the soil, but every time you test a seed, you plant it in a different garden.

The Experiment: The "Cosmic Splicing" Trick

To solve this, the researchers used a clever computer trick called splicing. Think of it like a digital "cut and paste" operation on the universe's blueprint.

The Setup: They took a specific "seed" (a dark matter halo) from one simulation. This seed had a specific density, speed, and gravitational pull.
The Swap: They cut out the "neighborhood" around that seed and replaced it with a completely different neighborhood.
- Scenario A: They placed the seed in an empty, lonely void.
- Scenario B: They placed the exact same seed right next to a massive, thick cosmic filament.
- Scenario C, D, E...: They moved it to various distances in between.

The Analogy: Imagine you have a specific recipe for a cake (the halo). You bake this cake in a quiet kitchen (isolated). Then, you take that exact same batter and bake it in a kitchen that is shaking violently because a giant earthquake (the filament) is happening next door. You want to see if the cake turns out different just because of the shaking, even though the recipe (the ingredients) stayed exactly the same.

What They Found

They ran this experiment on five different "Milky Way-sized" halos. Here is what happened:

1. The Cake's Size (Mass) Didn't Change

Result: Whether the halo was lonely or next to a giant filament, its mass and size stayed almost exactly the same.
The Takeaway: The "DNA" of the halo (its initial density) determines how big it gets. The neighborhood doesn't really matter for size. It's like the cake rises to the same height regardless of the earthquake; the recipe is the boss here.

2. The Cake's Spin and Shape (Orientation) Changed Wildly

Result: This is where things got crazy. The direction the halo spins and how it is shaped changed drastically depending on how close it was to the filament.

When the halo was far away, it spun one way.
When it was close to the filament, it spun a completely different way (sometimes flipping 90 degrees!).
The shape of the halo also twisted and turned.
The Takeaway: The "neighborhood" has a huge influence on the halo's orientation. The researchers found that the spin direction could fluctuate by up to 80% just by moving the halo closer to the filament.

Why This Matters

1. We Can't Predict Everything from the Start
For a long time, scientists thought they could predict how a galaxy would look just by looking at its initial conditions (its "birth certificate"). This paper proves that we can't. Even if you know the exact starting recipe, you can't predict the final spin or shape without knowing what the neighborhood looks like. The "environment" is a major player.

2. The "Cosmic Shear" Problem
Astronomers use a technique called "weak lensing" (like looking at the universe through a slightly warped glass) to map dark matter. They rely on knowing how galaxies align with the cosmic web. If the alignment is chaotic and unpredictable (as this paper suggests), it makes these maps much harder to read. It's like trying to read a map where the compass needle spins wildly depending on how close you are to a mountain.

3. The "Nature vs. Nurture" Balance
The paper concludes that for things like mass, "Nature" (the initial conditions) is the boss. But for things like spin and orientation, "Nurture" (the environment) is just as important, if not more so.

The Bottom Line

The universe isn't just a collection of isolated islands. The giant cosmic filaments act like a giant hand that grabs onto forming galaxies and twists them. You can't understand a galaxy's shape just by looking at its birth; you have to look at where it grew up.

The researchers successfully proved that non-linear effects (complex interactions that happen later in time) are just as powerful as the simple, linear rules we thought controlled the universe. It's a reminder that in the cosmos, context is everything.

1. Problem Statement

In the standard model of structure formation, the properties of dark matter (DM) halos are often predicted using linear perturbation theory applied to initial conditions (ICs). Theories like the Extended Press-Schechter formalism and Tidal Torque Theory (TTT) assume that halo properties (mass, spin, shape) are determined primarily by local initial density, tidal fields, and velocity fields within the halo's Lagrangian patch.

However, it remains unclear to what extent non-linear couplings between a forming halo and its large-scale cosmological environment (specifically proximity to massive filaments) alter these properties. Previous numerical studies using Gaussian Random Fields (GRFs) cannot disentangle intrinsic halo variations from environmental effects because every realization of the universe is different. This paper aims to isolate the causal impact of the large-scale environment on individual halos by holding the local initial conditions constant while systematically varying the external environment.

2. Methodology: Potential Field Splicing

The authors employ and extend the splicing technique (originally introduced by Cadiou et al. 2021b) to create a controlled numerical experiment.

Core Concept: The method involves taking a specific region (a Lagrangian patch) from one cosmological simulation (containing a target DM halo) and "splicing" it into the initial conditions of a different simulation (containing a large cosmological filament).
Novel Extension: Unlike previous implementations that only spliced the density field, this study extends the method to splice the gravitational potential field (and consequently the velocity field).
- By fixing the potential field within the patch, the authors ensure that the initial density, velocity, and tidal forces are identical across all simulations for the target halo.
- This effectively freezes the "linear" evolution of the halo, allowing any subsequent differences in halo properties to be attributed solely to non-linear couplings with the changing external environment.
Experimental Setup:
- Target: Five Milky Way-mass DM halos ( $1.7 - 3.5 \times 10^{12} M_\odot$ ).
- Environment: A massive cosmological filament.
- Procedure: For each halo, the authors generated 9 different initial condition sets. In each set, the halo's Lagrangian patch was spliced at incrementally increasing distances (0.75 Mpc steps) from the filament, covering a range of ~6 Mpc.
- Simulation: Dark Matter-only simulations were run using the RAMSES code (adaptive mesh refinement) from $z=75$ down to $z=0$ .
- Analysis: Halo properties (mass, spin, shape, alignment) were measured at $z=0$ and compared across the 9 positional variations for each halo.

3. Key Contributions

First Controlled Causal Test: This is the first study to explicitly decouple the linear evolution (fixed local ICs) from non-linear environmental effects, quantifying the "nature vs. nurture" debate for DM halos.
Potential Field Splicing: The authors successfully implemented a splicing method that fixes the potential field, ensuring that the initial tidal tensor and velocity field are perfectly preserved, which was not possible with density-only splicing.
Quantification of Non-Linear Coupling: The study provides a rigorous upper bound on the accuracy of analytical models (like TTT) that rely solely on local linear information to predict halo properties.

4. Key Results

The study analyzed the scatter in halo properties across the 9 environmental variations for each of the 5 halos. The results are categorized by the sensitivity of the property to the environment:

Mass and Virialization (Insensitive):
- Mass ( $M$ ) and Virial Radius ( $r_{vir}$ ): Showed negligible deviations (< 1%) across all environmental variations.
- Conclusion: The mass and size of a halo are almost entirely determined by the local initial density and tides (linear regime). Non-linear environmental coupling contributes less than ~1% to the final mass.
- Virialization ( $E_{kin}/E_{pot}$ ): Also showed minimal sensitivity.
Angular Momentum and Morphology (Moderately Sensitive):
- Spin Magnitude ( $|\mathbf{j}|$ ) and Spin Parameter ( $\lambda$ ): Showed fluctuations of up to 10–30% depending on the specific halo.
- Morphology (Axis ratios $b/a$ ): Showed significant scatter, with deviations reaching several times the numerical noise level.
- Conclusion: While the initial conditions set the bulk of the angular momentum, the accumulation process is modulated by non-linear interactions with the environment.
Orientation and Alignment (Highly Sensitive):
- Spin Orientation ( $\cos \theta_{J, e3}$ ) and Shape Alignment: These parameters exhibited the largest fluctuations, ranging from 10% to 80% around the mean value.
- Trend: Halos forming closer to the filament tended to have spins that were more perpendicular to the filament spine, while those further away showed different alignment trends.
- Significance: The scatter in orientation is comparable to, or even exceeds, the scatter seen in population-level statistics. This indicates that a large fraction of the observed alignment signal is driven by non-linear environmental coupling, not just linear tidal torques.

5. Significance and Implications

Limitations of Linear Theory: The results demonstrate that theories relying solely on local linear initial conditions (e.g., Tidal Torque Theory, Extended Press-Schechter) cannot accurately predict the properties of individual halos, particularly their orientation. These theories may capture population averages qualitatively but miss significant non-linear contributions.
Cosmic Shear Surveys (Euclid, etc.): The findings have direct implications for Intrinsic Alignment (IA) modeling in weak lensing surveys. Since halo orientation is heavily influenced by non-linear environmental effects, current IA models based on linear theory may be insufficient, potentially biasing cosmological parameter estimation.
Information Budget: The authors estimate that for halo mass, ~85% of the information is in the local density, ~14% in local tides/velocities, and only ~1% in non-linear environmental coupling. However, for orientation, the non-linear environmental contribution is dominant.
Future Directions: The authors propose that splicing techniques can be used to study how massive halos affect filament morphology (feedback loops) and plan to extend this work to include baryons to study galaxy formation and quenching in varying environments.

In summary, the paper establishes that while the "nature" (local initial conditions) dictates the mass and size of a dark matter halo, the "nurture" (large-scale environment) plays a critical, non-linear role in determining its spin orientation and shape, challenging the sufficiency of linear perturbation theory for individual halo predictions.